1. Field of the Invention
The present invention relates to an image reading apparatus and, more particularly, to an image reading apparatus for accurately reading or writing an image on an original surface by correcting an uneven light amount on the surface of a reading means, even when the magnification is changed, by intercepting the part of a light beams emanating from the image illuminated by an illuminating means by using light amount correcting members attached to the image forming lens (image forming means) via movable members. This type of image reading apparatus is suitably used is a copying machine.
2. Related Background Art
In conventional image reading apparatuses such as found is a scanning exposure copying machine capable of changing the copying magnification, the surface of an original placed on an original plate is illuminated by a bar-like light source such as a halogen lamp or a fluorescent lamp. The light beams reflected from the original surface are focused on the exposure surface of a photosensitive member by an image forming lens. Image information form the original is sequentially written on the exposure surface of the photosensitive member by changing the relative position of the original surface and the photosensitive member, i.e., by scanning.
Generally, the light amount on the edge of the image forming surface of an optical lens, such as when used as an image forming lens is attenuated in proportion to cos.sup.4 .theta., (where .theta. is the angle of incidence of the light). Accordingly, in an image reading apparatus using such an optical lens as a projecting lens, the illuminance on the exposure surface of a photosensitive member is high in its central portion and low in its peripheral portion. This uneven amount of light appears as an uneven density on the outputted image, or photocopy.
In some conventional image reading apparatuses, the illuminance distribution of a light source, or the width of a slit through which a reflected light beam from an original surface passes, is changed so that the edge of an image forming lens is brighter than its center. However, when copying is performed while the magnification is changed, the angle of view changes in accordance with the magnification change. This results in a nonuniform exposure surface illuminance distribution of a photosensitive member.
To prevent this, therefore, a light amount correcting plate which covers a wider area of the center of an image forming lens than the area of the edge of the lens is always placed at a fixed distance from the image forming lens. Consequently, an uneven light amount in the center and the edge of the image forming lens is corrected, and this makes the exposure surface illuminance of a photosensitive member uniform. In this method, the exposure surface illuminance can be made nearly uniform even if the angle of view changes when the magnification is changed.
Unfortunately, the effect of the light amount correcting plate can be obtained only when the plate is placed in a position where light beams from the center and the edge of the image forming lens separate to some extent. Usually, the effect of the light amount correcting plate can be obtained if the gap from the end face (lens surface) of the image forming lens is 30 to 40 mm. This light amount correcting plate is placed on the original surface side or the photosensitive member side of the image forming lens.
Recently, the image reading apparatuses as described above are under market pressure to have a wide zoom magnification range and a small size.
For example, in a so-called mirror zoom type image reading apparatus which uses a single-focus lens as an image forming lens and changes the magnification by moving the lens and reflecting mirrors to predetermined positions, the moving amounts of the lens and the reflecting mirrors increase as the zoom range widens. In a 6-mirror image reading apparatus in which a first reflecting mirror, and a second through a sixth reflecting mirror are each arranged in this order from the original surface side, the magnification is changed by changing the total optical path length by moving the fourth and fifth reflecting mirrors. When equal-magnification copying or enlarged copying is performed in an image reading apparatus of this type, the gap between the third reflecting mirror and the lens narrows during full scan. Also, the lens and the fourth reflecting mirror move closer to each other upon minimum reduction.
If the aforementioned method of placing the light amount correcting plate at a fixed distance from the lens is used to obtain uniform exposure surface illuminance of the photosensitive member, the light amount correcting plate and the third reflecting mirror unavoidably interfere with each other when equal-magnification copying is performed. Alternatively, the light amount correcting plate and the fourth reflecting mirror unavoidably interfere with each other when reduced copying is performed.
In the conventional mirror zoom type image reading apparatus, therefore, a gap of at least 30 to 40 mm is formed from the end face (lens surface) of the image forming lens as a space for placing the light amount correcting plate to avoid the interference between the light amount correcting plate and the reflecting mirrors. However, this increases the size of the apparatus because the gap of at least 30 to 40 mm is formed from the end face (lens surface) of the image forming lens.